Methane is available in great quantities in natural gas. The composition of natural gas varies with the source but essentially it is made up of methane (typically about 75% by weight), ethane, propane, and other paraffinic hydrocarbons, along with small amounts of inorganic gases. The chief use of methane is as a fuel, but processes are known for converting it to higher molecular weight products. For example, methane can be first converted to methyl halide and then catalytically condensed to hydrocarbons having two or more carbon atoms to the molecule. Such a process is described in Gorin et al U.S. Pat. No. 2,488,083. Modern processes convert methane to ethylene, acetylene, hydrogen and high surface area carbon by high temperature pyrolysis. Carbon produced by the process, although economically valuable, presents costly and sometimes difficult handling and disposal problems. If the methane could be converted to primarily gaseous or liquid materials, conversion could be accomplished at the well site so that one could ship product rather than methane. Shipment from the well site would therefore be less hazardous, less costly and have higher value. By converting the methane to ethane, and/or ethylene, it would have great value as a petrochemical feedstock for the production of ethylene oxide, ethylbenzene, ethylchloride, ethylene dichloride, ethyl alcohol and polyethylene from which are manufactured hundreds of end products.
The present invention provides a process for the conversion of methane to saturated and unsaturated hydrocarbons. It is a one-step process using chlorine gas as a recyclable, active catalyst and is simple, economical and readily usable at the well site. It can be operated so as to produce a desired mix of the hydrocarbons and can also produce hydrogen.
Specifically, a method is provided for converting methane into at least one higher molecular weight hydrocarbon, which method comprises reacting a mixture of chlorine and a gas comprising methane in specific ratios and under specific temperature conditions. In particular, the methane and chlorine is used in a mole ratio of about 1:1 to 10:1 under conditions to provide a reaction temperature of at least 700.degree. C., preferably 700.degree.-1710.degree. C. The result is the formation of hydrogen chloride with varying quanitities of hydrogen and saturated and unsaturated hydrocarbons, notably ethane and ethylene.
The methane and chlorine gases are mixed together and ignited in a reaction vessel. The composition of the resultant product can be controlled by varying the ratio of the reactants, the temperature and/or the pressure within the reaction vessel. While small amounts of higher homologues can also be produced, with regard to the production of ethane, ethylene and hydrogen, the reaction proceeds in accordance with the general equation: EQU 2CH.sub.4 +(1+y)Cl.sub.2 .fwdarw.(2y+2)HCl+(1-x)C.sub.2 H.sub.6 +xC.sub.2 H.sub.4 +(x-y)H.sub.2
wherein x is from 0 to 1, y is from 0 to 1 and x is greater than y. Again, with regard to the production of ethane and ethylene, when y is 0 the process is stoichiometric with respect to methane consumption and it is preferred to operate the process with a mole ratio of methane to chlorine of at least 2:1. This gives rise to the simplified equation: EQU 2CH.sub.4 +Cl.sub.2 .fwdarw.2HCl+(1-x)C.sub.2 H.sub.6 +xC.sub.2 H.sub.4 +xH.sub.2
wherein x is from 0 to 1.
By operating with at least a stoichiometric ratio of methane to chlorine, one avoids the danger of contamination with polychlorinated end product. An inspection of the above formula reveals that the main products of the reaction will vary and include ethylene, hydrogen and ethylene, ethane, and mixtures thereof. The value of x, i.e., the composition of the product, can be controlled by controlling the pressure within the reaction vessel and the temperature of reaction. The temperature in turn can be controlled by increasing the relative amount of methane mixed with the chlorine, or by adding water to the reaction mixture, or physically by external cooling of the reaction chamber. The pressure can be controlled by appropriate valving of reactant and product streams or by allowing the mixture to do recoverable work such as by expansion.
Using well known methods, the hydrogen, hydrogen chloride and excess methane can be separated from the other products. The methane can be recycled or it and the hydrogen can be used to provide energy for the system. Hydrogen of course can also be shipped for use in other processes. The hydrogen chloride can be burned in air to reform the initial chlorine which can then be recycled to constitute the reaction mixture.